1. Field of the Invention
The present invention relates to a motor protective device, and more particularly to a motor protective device which protects a motor used for electrically-operated power steering, for example, by making use of a motor overheat prevention function.
2. Description of Background Art
With respect to the steering of a vehicle by rotating a steering shaft, an electrically-operated power steering system is known which eases the steering by imparting a rotational auxiliary force to the steering shaft by using an electrically-operated motor.
In JP-A-2005-324796, a control device is disclosed wherein an electrically-operated power steering device is provided for preventing the overheating of the electrically-operated motor. The electrically-operated power steering device estimates a temperature of the wiring of a motor and performs a motor temperature protective control based on the estimated temperature.
In general, in estimating the wiring temperature of the motor, a current value which flows in the wiring and a resistance value of the wiring are used in accordance with Joule's Law. More specifically, assuming the current value is I, the resistance value is R and an electricity supply time is t, a heating value Q can be estimated by a following formula (1).Q=I×I×R×t  (1)
Although the heating value is estimated based on this formula (1), to further enable the estimation of the temperature, it is necessary to take also a radiating value into consideration. A following formula (2) is a heating value estimation formula which contains a constant a as a radiating value correction term. A cumulative value T represents a temperature.Cumulative value T=Σ(K×I×I−a)  (2)
This formula (2) is an estimation formula for estimating the temperature by cumulating the heating value when the power steering is operated and the electricity is supplied to the electrically-operated motor for an electricity supply time, wherein the constant a is subtracted as the radiating value. The constant a in the formula (2) is, to ensure the temperature protection by estimating a high temperature, set to an extremely small value such that the cumulative value returns to zero with a time longer than a time that the temperature of the wiring returns to a normal temperature when the supply of electricity is stopped from the maximum temperature. When the constant a is set to an excessively large value, the cumulative value T tends to be decreased thus facilitating the estimation of the wiring temperature at a low value. When the supply of electricity is not performed for a long time, the cumulative value T returns to zero due to the constant a. Here, in the formula (2), a coefficient K is a cumulating coefficient and is a numerical value obtained by an experiment carried out preliminarily so as to approximate a calculated value to an actually measured value.
With the use of the above-mentioned formula (2), the temperature of the electrically-operated motor can be estimated without using the temperature sensor and it is possible to protect the electrically-operated motor by stopping the supply of electricity to the electrically-operated motor when the estimated temperature becomes the preset temperature or more.
Although the above-mentioned formula (2) is suitable for a vehicle which exclusively travel on a paved general road, the formula (2) is not always suitable for an all terrain vehicle (ATV) which travels on an off-road or the like. In the off-road traveling, a load on power steering is large and the frequency of the supply of electricity to the electrically-operated motor is increased. Thus, the cumulative value T becomes excessive. In an actual operation, there may be a possibility that the supply of electricity to the electrically-operated motor is stopped at a low temperature which requires no protection for generation of heat and the auxiliary force is not imparted to the steering shaft.
The reason that the cumulative value T becomes excessive and is no longer correlated with the actual temperature is as follows. More specifically, although the constant a for correction which is decided by taking the radiating value into consideration is an extremely small fixed value, the actual radiating value is changed due to the difference between the temperature of the electrically-operated motor and the ambient temperature. When the motor is operated for a long time, the temperature difference becomes large. Thus, the radiating value is increased whereby the actual temperature of the electrically-operated motor is elevated to a small extent. Accordingly, when the formula (2) in which the constant a takes the extremely small value is adopted, the cumulative value of the heating value tends to be increased and it is considered impossible to take a correlation between the cumulative value T and the actual temperature.